This invention relates to a feedstock injector for receiving a stream of heated gas, and for injecting feedstock material axially into the downstream flow of the heated gas.
Thermal spraying is a coating method wherein powder or other feedstock material is fed into a stream of heated gas produced by a plasmatron or by the combustion of fuel gasses. The feedstock is entrapped by the hot gas stream from which it receives heat and momentum and it is further impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the cladding of subsequent thin layers or lamellae.
It has been recognized for some time that, in the case of some thermal spray applications, injecting feedstock axially into a heated gas stream presents certain advantages over traditional methods wherein feedstock is fed into the stream in a direction generally described as radial injection, in other words in a direction more or less perpendicular to the direction of travel of the stream. Such advantages of the axial injection relate mainly to the potential to control better the linearity and the direction of feedstock particle trajectory. It would be therefore desirable to inject feedstock in a manner that induces an optimal particle trajectory in the axial direction.
Plasma torches with axial injection of feedstock can be classified in two major groups: a) those with multiple cathodes, also known as the pluri-plasmatron or the multiple-jet type; and b) those with a single cathode, also known as the single stream type.
Examples of multiple cathode plasma torches with axial injection are found in U.S. Pat. No. 3,140,380 of Jensen, U.S. Pat. No. 3,312,566 of Winzeler et al., U.S. Pat. No. 5,008,511 of Ross and U.S. Pat. No. 5,556,558 of Ross et al. They show a plurality of plasmatrons symmetrically arranged about the axis of the plasma spray torch and provide for nozzle means to converge the plurality of plasmas into a single plasma stream. Feeding means are also provided to inject feedstock materials along the axis of the single plasma stream. Although such plasma torches can produce satisfactory coatings, they involve complex torch configurations as well as the use of multiple power supplies for powering the multiple cathodes. The use of multiple cathodes and multiple arc chambers, which need to be replaced regularly, induce high operating costs for such plasma torches. A different approach to achieve axial injection employing multiple cathodes and a complex single arc chamber configuration is found in U.S. Pat. No. 5,225,652 and U.S. Pat. No. 5,332,885, both issued to Landes.
The single cathode type plasma torches with axial injection have certain advantages such as less complex torch configuration, less operating costs and less manufacturing costs for the plasma system. It has been recognized for some time that the introduction of powder axially through a central hole in the cathode tip is not an efficient solution for axial injection. Such an approach is found in U.S. Pat. No. 5,225,652 of Landes. The powder interferes with the electric arc, readily resulting in the malfunctioning of the torch. Other arrangements for the single cathode approach are found in U.S. Pat. No. 4,540,121 of Browning, U.S. Pat. No. 4,780,591 of Bernecki et al., U.S. Pat. No. 5,420,391 of Delcea and U.S. Pat. No. 5,837,959 of Muehlberger et al. For example, Muehlberger et al. teach an output plasma nozzle oriented at an acute angle with respect to the torch axis. A powder feed tube axial with the output nozzle opens at or about the bent in the plasma path or alternatively penetrates into the nozzle and the plasma stream. Both alternatives proposed by Muehlberger induce a non-uniform interaction between the plasma stream and the powder due to bending of the stream and the introduction of an angled tube in the path of the stream. The plasma stream has a lower density and velocity along the wall of the far side bent, which affects the trajectory of the powder. Bernecki et al. teaches semi-splitting of the plasma stream by means of an arm which protrudes radially into the plasma stream and connects to a core member positioned axially within the plasma torch nozzle. The feedstock is injected axially through the core member. This approach creates an asymmetrical plasma stream at the point of powder injection, with a portion of the plasma stream going undisturbed about the injector, while the rest of the stream is split by the arm before the injection point.
U.S. Pat. No. 5,420,391 of Delcea teaches single step splitting of a plasma stream and, similar to Bernecki, the feedstock is injected axially through a core member. The plasma is split only once at the upstream end of the splitting channels. This approach has practical disadvantage with respect to the ability to efficiently converge and accelerate discrete plasma streams about the injection tip. If more than two splitting arms are provided in Delcea ""391 in an attempt to more uniformly distribute the streams about the injection point and to achieve an acceleration of the split gas streams, the thermal efficiency of the torch would be impaired due to the full exposure of the arms to the flow of hot gas. One of the disadvantages common to the designs found in Bernecki ""591 and Delcea ""391 is related to the short length of the feedstock input passage running axially through the respective cores. When using reasonable carrier gas flows, the carrier gas and the powder are bent at 90xc2x0 and cannot be accelerated sufficiently along the short feedstock passage in order to be efficiently projected axially into the plasma stream without being affected by turbulence. If higher carrier gas flows are used to more efficiently push the powder axially, the injection of the carrier gas will cool the plasma to the detriment of torch efficiency. On the other hand, if the feedstock input passage is extended, the elongated core and the corresponding interconnecting arms, but in particular the downstream portion of the core become exposed excessively to the hot plasma, with deleterious effects on the core and on the thermal efficiency of the torch. Further, if the core is elongated in Delcea ""391, the angle of convergence shifts further downstream relative to the feedstock injector point thereby resulting in inefficient axial injection.
U.S. Pat. No. 4,540,121 of Browning discloses a plasma torch, which splits the plasma stream into a first plurality of streams and then further splits each of the first plurality of streams into a second plurality of streams while at the same time bending the streams at 90 degrees. By the very nature of this design, the second splitting occurs in an asymmetrical and non-coaxial manner. The inlets of the second splitting channels open into the side wall of each first splitting channel thereby receiving unequal gas flows i.e. due to the gradient in gas velocity and pressure, i.e. the upstream located channels receive more gas flow than the further downstream located channels. This results in an unbalanced, asymmetrical flow convergence about the feedstock injection duct thereby inducing non-axial trajectories for the feedstock particles. Further, the Browning torch has a complex configuration. Consequently, the torch will have a low thermal efficiency due to excessive exposure of the internal walls and pathways to the hot plasma gas and due to the multiple turbulent disruptions of the plasma stream induced by the combination of multiple splitting and bending of the streams.
With respect to combustion spray torches, in a majority of cases the powder is injected radially at the inlet of an elongated output nozzle. In U.S. Pat. No. 4,416,421 of Browning, the powder is injected axially in a flame-spray apparatus similar to the plasma torch described by Browning in U.S. Pat. No. 4,540,121. Therefore, the feedstock injection method described by Browning in Patent ""421 presents the same disadvantages as described above with reference to the Browning Patent ""121.
In the case of thermal spray torches, it is well known practice to attach an output spray nozzle in order to increase feedstock velocity and the transfer of heat to the feedstock. As a general rule, the longer the output nozzle the more velocity is transferred from the gas stream to the feedstock and therefore denser thermal spray coatings can be obtained. One of the main factors that limit the size of the output nozzle is the trajectory of the molten feedstock along the nozzle passage. If the injection of the feedstock is such that at least some feedstock deviates towards the internal wall of the nozzle, it will solidify and build up on the cold surface of the wall resulting in a malfunctioning of the spray process.
Accordingly, it would be desirable to provide a superior feedstock injector for attachement to a single stream thermal spray torch, the injector providing for optimal interaction between the feedstock and the gas stream and between the gas stream and the internal pathways of the injector.
In one embodiment of the invention a feedstock injector having a longitudinal axis includes a first plurality of channels each having an upstream end and a downstream end, the first plurality of channels extending from the upstream end to an intermediate region of the injector, the fist plurality of channels disposed symmetrically about the longitudinal axis of the injector and shaped at their inlet ends to receive a stream of gas and to split the stream into a fist plurality of streams, each first plurality of channels having an outer wall and an inner wall, the plurality of inner walls substantially defining a first core segment therebetween. The injector also includes a second plurality of channels each having an upstream end and a downstream end, the second plurality of channels extending from the intermediate region towards the downstream end of the injector, the second plurality of channels disposed symmetrically about the longitudinal axis and comprising at least two channels for each channel of the first plurality of channels, the upstream ends of the second plurality of channels connected to the downstream ends of the first plurality of channels and shaped to receive the streams of gas flowing through the first plurality of channels and to split the streams into a second plurality of streams, each channel of the second plurality of channels having an outer wall and an inner wall, the plurality of inner walls substantially defining a second core segment therebetween. The injector further includes a feedstock input passage opening at the downstream end of the second core segment and oriented to direct feedstock axially in the downstream direction.